The multi-antenna technology is an important constitutional portion of the fourth generation wireless communication such as LTE-advanced. One important application in the multi-antenna technology is the closed-loop downlink pre-coding. In such application, a client (terminal) measures a downlink channel from a base station to the client. The client then proposes a corresponding pre-coding matrix to the base station. This proposition is sent to the base station via a feedback link. Finally, in a downlink transmission, the base station may adopt directly the proposed pre-coding matrix.
In the above procedure, one important step is how to feed back the proposed pre-coding matrix. The most commonly used means is a manner of codebook based feedback. This manner first defines one codebook containing multiple pre-coding matrixes. The codebook is known for both the base station and the client. If the client proposes a certain pre-coding matrix in the codebook, an index corresponding to the pre-coding matrix is fed back to the base station. For instance, a certain codebook contains four pre-coding matrixes, which are indexed, each by two bits, as {00, 01, 10, 11}. If the client proposes the base station to use the second pre-coding matrix, the bits {01} are sent to the base station. The base station thereby selects the pre-coding matrix corresponding to the bits {01} to send signals. Such manner of pre-coding matrix index (PMI) based feedback can achieve a better performance in reducing the feedback overhead and assuring a high downlink throughput and so on.
There is another important subject in the fourth generation wireless communication system standardization is support for downlink multi-antenna such as eight emission antennas. In a specific implementation, the eight downlink emission antennas are located at an appropriate position of a base station which emits signals to be transmitted to a receiving end (terminal) via the eight emission antennas. One important implementation for eight emission antennas is eight dual-polarized antennas.
FIG. 1 is a diagram showing the configuration of the eight dual-polarized antennas.
The configuration of the dual-polarized antenna is shown as FIG. 1, in which antennas a, b, c and d are polarized at positive 45 degree while antennas a′, b′, c′ and d′ are polarized at negative 45 degree, and respective pairs of antennas a and a′, b and b′, c and c′ as well as d and d′ are respectively located at the same physical position. Since the space between the dual-polarized antennas is usually half-wavelength, i.e. 0.5λ, the antennas a, b, c and d have a certain correlation, and the antennas a′, b′, c′ and d′ have a certain correlation. Meanwhile, a first antenna array (including the antennas a, b, c and d) and a second antenna array (including the antennas a′, b′, c′ and d′) are independent from each other.
FIG. 2 is a diagram showing equivalent channels for the eight dual-polarized antennas.
Actually, the channels of the eight dual-polarized antennas can be equivalently considered as two antenna arrays as shown in FIG. 2, in which each of the antenna arrays contains four antennas (the antennas a, b, c and d as well as the antennas a′, b′, c′ and d′ respectively). The channels of four antennas of one antenna array are correlated channels, while the channels of the two antenna arrays are independent from each other.
In the eight dual-polarized antennas as described above, a client may receive signals from the two antenna arrays. Since a basic codebook design manner is to feed back the index of the pre-coding matrix for a single antenna array, there are indexes corresponding to two pre-coding matrixes required to be fed back to the base station, wherein each of the pre-coding matrixes corresponds to one antenna array. Furthermore, the phase offset between the two pre-coding matrixes is also required to be fed back to the base station such that the signals of the two antenna arrays can be combined coherently at the client. Therefore, the feedback with respect to the eight dual-polarized antennas as shown in FIG. 1 and FIG. 2 may include three portions:
1) the index corresponding to a pre-coding matrix of the first four-antenna array;
2) the index corresponding to a pre-coding matrix of the second four-antenna array; and
3) the phase offset between the channels of the two antenna arrays.
FIG. 3 (A) and FIG. 3(B) are respectively schematic diagrams showing the comparison of four-antenna feedbacks based on a double codebook and a single codebook.
A general means for feeding back the index of a pre-coding matrix for a four-antenna array is a method employing a single codebook, as shown in FIG. 3 (A). In the method as shown in FIG. 3 (A), one codebook is used to represent pre-coding matrixes of one antenna array (directions of respective antenna channels), wherein, for example, the index corresponding to the pre-coding matrix is formed by four bits (represented by four blocks), and the feedback is performed from a terminal to a base station in a short period (e.g. 10 ms). In FIG. 3 (A), the directions of different pre-coding matrixes of the antenna array of the base station are represented by positions of light colored circles.
Another known means for feeding back the index of a pre-coding matrix for a four-antenna array is a method employing a double codebook, as shown in FIG. 3 (B). In the method as shown in FIG. 3 (B), the four antennas in the antenna array are assumed to experience channels with high spatial correlation, so that instantaneous channel directions represented by the positions of light colored circles in FIG. 3 (B) generally fluctuate around an average (long-term) channel direction represented by the position of a deep colored circle in FIG. 3 (B). In the double codebook design, the above four bits are divided into two groups. The former two bits are used to constitute indexes of pre-coding matrixes of the first codebook for feeding back a long-term channel direction, which is referred to as a long-term pre-coding matrix index later. The latter two bits are used to constitute indexes of pre-coding matrixes of the second codebook for feeding back the difference between the long-term channel direction and the instantaneous channel direction which is referred to as an instantaneous pre-coding matrix index. Since the variation of the long-term channel direction is slow, a long period (for example 100 ms) can be used to feed back it. However, the variation of the difference between the instantaneous channel direction and the long-term channel direction is fast, thus a short period (for example 10 ms) is required to feed back it. Therefore, the employment of the double codebook method can effectively reduce feedback overhead or increase feedback accuracy.
The double codebook method may be directly applied to the eight dual-polarized antennas, and the corresponding feedback includes:
1) a long-term pre-coding matrix index and an instantaneous pre-coding matrix index corresponding to a first antenna array;
2) a long-term pre-coding matrix index and an instantaneous pre-coding matrix index corresponding to a second antenna array; and
3) the phase offset between the channels of the first antenna array and the second antenna array.
FIG. 4 is a diagram showing the pre-coding matrix indexes corresponding to the two antenna arrays respectively.
Here, it is defined that a first pre-coding matrix index corresponding to the first antenna array includes a first long-term pre-coding matrix index (indicated by the former two bits) and a first instantaneous pre-coding matrix index (indicated by the latter two bits), and a second pre-coding matrix index corresponding to the second antenna array includes a second long-term pre-coding matrix index (indicated by the former two bits) and a second instantaneous pre-coding matrix index (indicated by the latter two bits). Phase offset is generally considered as instantaneous information, and it is fed back in a short period. Here, the phase offset between the two pre-coding matrixes is indicated for example by two bits (represented by two blocks). Wherein, the first long-term pre-coding matrix index and the second long-term matrix index are fed back using long periods, and the first instantaneous pre-coding matrix index, the second instantaneous matrix index and the phase offset are fed back using short periods.